Semiconductor device and manufacturing method thereof

ABSTRACT

A deep trench is formed in a silicon substrate. The inner surface of the trench is next coated with a thin polycrystalline silicon film (liner film) so as not to close the trench. A silicon germanium film (node electrode) is then formed on the thin polycrystalline silicon film so as not to close the trench. Next, a heat treatment is performed on the silicon germanium film thereby to flow only the silicon germanium so that the trench is filled.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 11-075080, filed Mar. 19,1999, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] In accordance with high integration of semi-conductor integratedcircuits represented by DRAMs, the device area has been reduced fromgeneration to generation. In a DRAM in which each memory cell comprisesone transistor and one capacitor, reduction of the device area involvesreduction of the area of capacitors for storing information, so thefunction of storing information is deteriorated.

[0003] Hence, in a DRAM, various countermeasures for maintaining asufficient capacity for the capacitors have been taken so that theinformation storage function might not be deteriorated by reduction ofthe device area. Adoption of a capacitor having a three-dimensionalstructure is one of those countermeasures.

[0004] A trench capacitor is known as one of capacitors of this kind. Itis important for the trench capacitor to bury a deep trench withoutcausing a void or a seam. FIGS. 6A to 6D are sectional views showingsteps of a method for manufacturing a conventional trench capacitorwhich was proposed to achieve this object (Jpn. Pat. Appln. KOKAIPublication No. 10-56154.)

[0005] According to this conventional method, as shown in FIG. 6A, amask pattern 82 is firstly formed on a silicon substrate 81 and thesilicon substrate 81 is etched with the above mask pattern used as amask by a RIE (Reactive Ion Etching) method, thereby to form a deeptrench 83 in the silicon substrate 81. A layered film consisting of asilicon oxide film, a silicon nitride film, and a silicon oxide film canbe used for the mask pattern 82.

[0006] Next, as shown in FIG. 6B, a collar insulating film 84 is formed,and thereafter, a capacitor insulating film 85 is deposited on theentire surface of the deep trench 83. The collar insulating film 84 isformed as follows.

[0007] The inner surface of the trench 83 is oxidized thermally to forma thin oxide film (not shown). Next, a resist (not shown) is applied soas to fill the trench 83 after a silicon nitride film (not shown) havingthickness of approximately 10 nm is deposited on the entire surface by aLPCVD method. The resist at an upper portion of the trench is exposedand developed. As a result, an inner wall of the upper portion of thetrench is exposed. Next, the resist is peeled after the silicon nitridefilm and the silicon oxide film at the upper portion of the trench by aCDE method. Further, a side surface of the upper portion of the trenchis selectively oxidized thermally with the silicon nitride film at abottom portion of the trench used as a mask, thereby to form a collaroxide film 84. Next, the silicon nitride film at a lower portion of thetrench is peeled by using HF/glycerol. Finally, the silicon oxide filmat the lower portion of the trench is removed.

[0008] Next, as shown in FIG. 6C, a thin polycrystalline silicon film 86and an amorphous silicon film 87 containing impurities such asphosphorus or the like are deposited sequentially on the entire surfaceso as not to close the trench 83. The thickness of the thinpolycrystalline silicon film 86 is, for example, 20 nm. The thinpolycrystalline silicon film 86 is used as a liner film and theamorphous silicon film 87 is used a node electrode.

[0009] Finally, as shown in FIG. 6D, the amorphous silicon film 87 issubjected to a heat treatment so that the amorphous silicon film 87flows to fill the trench 83.

[0010] This method is to fill the deep trench 83 with the amorphoussilicon film 87 without causing any voids or seams, by utilizing thepoint that the amorphous silicon tends to move more easily than thepolycrystalline silicon.

BRIEF SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a semiconductordevice and a method of manufacturing the same having a buried structure(in which a conductive film is buried in a deep trench) which can berealized easily.

[0012] To achieve the above object, a semiconductor device according tothe present invention is comprises: a silicon substrate having a trenchformed in a surface of the silicon substrate; a polycrystalline siliconfilm coating an inner surface of the trench such that the trench is notclosed; and a conductive film made of material having a lower meltingpoint than silicon and formed on the polycrystalline silicon film so asto fill the trench.

[0013] Further, a method of manufacturing a semiconductor device,according to the present invention, comprises steps of: forming a trenchon a surface of a silicon substrate; coating an inner surface of thetrench with a polycrystalline silicon film as a liner film such that thetrench is not closed; forming a conductive film made of material havinga lower melting point than silicon, on the polycrystalline silicon film,such that the trench is not closed; and flowing the conductive film soas to fill the trench, by performing a heat treatment on the conductivefilm.

[0014] The pressure during the heat treatment is preferably set higherthan the pressure at which the conductive film is formed. Further, aheat treatment is preferably performed on the conductive film in a statethat no oxide film exists on the surface of the conductive film.

[0015] In the present invention, when a trench is buried by a conductivefilm with a polycrystalline silicon film (liner film) insertedtherebetween, the conductive film has a lower melting point than thepolycrystalline silicon film. Therefore, in the step of flowing theconductive film by a heat treatment thereby to fill the trench (reflowstep), the polycrystalline silicon film and the conductive film can beprevented from flowing integrally. Accordingly, a deep trench can beburied easily with a conductive film having an excellent buried form.

[0016] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0017] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0018]FIGS. 1A to 1D are sectional views showing steps of a method formanufacturing a trench capacitor, according to an embodiment of thepresent invention.

[0019]FIGS. 2A and 2B are sectional views showing shapes of a trenchbefore and after a heat treatment.

[0020]FIGS. 3A to 3C are views explaining characteristics of shapes of atrench before and after a heat treatment.

[0021]FIG. 4 is a view showing a relationship between the resistancerate of a silicon germanium film applied with boron or phosphorus andthe germanium ratio.

[0022]FIG. 5 is a view showing the dependency of the melting point ofSi_(X)Ge_(1-X) (0≦x≦1) on the silicon ratio.

[0023]FIGS. 6A to 6D are sectional views showing steps of a conventionalmethod for manufacturing a trench capacitor.

[0024]FIG. 7 is a sectional view explaining a problem of theconventional manufacturing method.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Explained first will be a problem of a conventional method shownin FIGS. 6A to 6D which has been found by the present inventors.

[0026] According to studies by the present inventors, it has beenclarified that a poly-crystallized amorphous silicon film 87 and a thinpolycrystalline silicon film 86 flow integrally in the heat treatmentstep shown in FIG. 6D and clearances 88 ₁ to 88 ₄ are formed as a resultin the trench 83 as shown in FIG. 7. Particularly, if the aspect ratioof the trench 83 is as high as 10 or more, it is difficult for theconventional method to bury the trench 83 with a silicon film withoutcausing clearances 88 ₁ to 88 ₄.

[0027] The clearances 88 ₁ to 88 ₄ of this kind cause a problem ofincrease in resistance of the amorphous silicon film 87 (noderesistance). Further, the clearance 88 ₁ formed at an upper portion ofthe trench 83 as shown in the figure acts to separate the upper portionof the amorphous silicon film 87 from the other portions. Consequently,a voltage of a predetermined level cannot be applied to the entireamorphous silicon film 87. In other words, the voltage of apredetermined level is applied to only a part of the amorphous siliconfilm 87. A problem hence arises in that the clearance 88 ₁ reduces astored charge.

[0028] Embodiments of the present invention capable of solving thisproblem will now be explained with reference to the drawings below.

[0029]FIGS. 1A to 1D are sectional views showing steps of a method ofmanufacturing a trench capacitor according to an embodiment of thepresent invention.

[0030] As shown in FIG. 1A, a mask pattern 2 is formed on a siliconsubstrate 1, and the silicon substrate is etched by the RIE method withthe above mask pattern 2 used as a mask thereby to form a deep trench 3in the silicon substrate 1.

[0031] A layered film consisting of a silicon oxide film, a siliconnitride film, and a silicon oxide film is used for the mask pattern 2.The trench 3 is 6 μm deep and has an aspect ratio of 10 or more.

[0032] In this case, a preferable method of forming the trench 3, i.e.,the method of forming a trench 3 which has a shape with a round bottomportion as shown in FIGS. 1A to 1D will be as follows.

[0033] That is, after forming the trench 3 in a well-known method, thesilicon substrate 1 is processed with a hydrogen fluoride aqueoussolution to remove a natural oxide film not shown. Subsequently, thesilicon substrate 1 is introduced into a chamber, and a heat treatmentat 1000° C. for 120 seconds is carried out under condition that onlyhydrogen flows at 20 slm and the pressure is set to 80 Torr.

[0034] At this time, a natural oxide film which is formed again on thesurface of the silicon substrate 1 until the silicon substrate 1 isintroduced into the chamber is removed, so silicon on the surface of thesubstrate is exposed. Therefore, such surface diffusion of silicon thatminimizes the surface area occurs at the substrate surface. Due to thissurface diffusion, the shape of the opening of the trench 3 after theheat treatment becomes such a shape (elliptical shape) that is definedby deforming the shape (rectangular shape) of the opening of the trench3 immediately after formation thereof so as to increase the minimumvalue of its radius of curvature. As a result, the bottom portion of thetrench 3 is rounded.

[0035] Further, according to studies made by the present inventors, ithas been found that irregularity caused by RIE on an inner surface ofthe trench 3 is eliminated by the heat treatment. Therefore, the innersurface of the trench 3 comes to have a smooth shape having littleroughness after the heat treatment.

[0036] Due to this heat treatment, the shape (a rectangular shape) ofthe trench 3 at the time of formation is changed into such a shape (anelliptic shape) that increases the minimum value of the radius ofcurvature, and the irregularity of the inner surface of the trench 3caused by the RIE is eliminated, so concentration of electric fields canbe relaxed. As a result, the withstand voltage can be improved.

[0037] The present inventors have investigated the trench 3 before andafter deforming the shape of the trench by the heat treatment under adecompressed condition as described above by a cross-sectional SEM. As aresult, as shown in FIG. 2A, it was found that the shape of trench 3becomes narrower as the RIE proceeds deeper. However, as shown in FIG.2B, it was also found that the trench 3 is deformed to have such aregion in which the cross-sectional area is larger at a position closerto the bottom in the bottom portion of the trench. Owing to thisdeformation, it is possible to obtain securely a larger capacitor areathan that of a conventional device.

[0038] Further, according to studies made by the present inventors, ithas been also found that the following is given if the shape of thetrench 3 is deformed by the heat treatment under the decompressedcondition as described above. Suppose that a first cross-section C1 is across-section where the trench 3 is cut along a first plane P1 having anormal line parallel to the depth direction of the trench, at a positiondistant by 4D/5 (D is the depth of the trench 3) from the bottom of thetrench 3, and that a second cross-section C2 is the cross-section wherethe trench 3 is cut along a second plane P2 having a normal lineparallel to the depth direction of the trench 3, at a position distantby D/5 from the bottom of the trench 3, as shown in FIGS. 3A to 3C. Avalue R2 which is obtained by dividing the major axis diameter of thesecond cross-section C2 by the minor axis diameter thereof is smallerthan 1.1 times the value of R1 which is obtained by dividing the majoraxis diameter of the first cross-section C1 by the minor axis diameterthereof.

[0039] In this case, a hydrogen fluoride aqueous solution is used for apreliminary treatment (removal of a natural oxide film). However, incase where a thin oxide film of approximately 1.5 nm is formed with amixed solution of hydrochloric acid water and ozone water, the thinoxide film can be removed together with the natural oxide film by alater heat treatment in the chamber. It is therefore possible to obtainthe same effect as in the case of using the hydrogen fluoride aqueoussolution.

[0040] The pressure and temperature in the chamber are set to 80 Torrand 1000° C. during the heat treatment for deforming the shape of thetrench. However, since surface diffusion of silicon occurs at 850° C. ormore under a decompressed condition, the shape of the trench can bedeformed likewise in this case.

[0041] The surface diffusion of silicon appears more clearly and thetrench 3 causes larger deformation of shape as the pressure and thetemperature respectively become lower and higher. Further, if thetemperature is higher than 1200° C., there occurs a phenomenon that thebottom portion of the trench 3 is split. Therefore, the temperature ofthe heat treatment for causing surface diffusion of silicon must be setto 1200° C. or less.

[0042] It has been known that the surface diffusion of silicon issuppressed by a heat treatment in a gas atmosphere in which a hydrogengas and a PH₃ gas are let flow simultaneously. Accordingly, in casewhere only the PH₃ gas is let flow first and only the hydrogen gas islet flow next, the shape of the trench does not change while doping Pand the shape obtained at the time point when only the hydrogen gas islet flow is maintained even after the doping of P.

[0043] The above explanation has been made of a case where a hydrogengas is let flow to cause surface diffusion of silicon. However, it isnot always necessary to flow a hydrogen gas, and a flow phenomenonoccurs as long as the pressure is reduced.

[0044] To cause a flow effectively, it is preferable that the surface ofthe silicon substrate 1 is oxidized and a heat treatment is carried outin an environment that the partial pressures of an oxygen gas and awater vapor which act to suppress surface diffusion of silicon are low.

[0045] However, if a hydrogen gas is let flow as described above, thereaction that silicon is oxidized into SiO₂ is suppressed and tends todecrease due to deoxidation action of hydrogen, and therefore, siliconcan easily be flowed.

[0046] That is, even in an environment that the partial pressures of theoxygen gas and the water vapor are not low, surface diffusion of siliconis possible if deoxidation proceeds in oxidation-deoxidation reactionbetween silicon and SiO₂.

[0047] Next, as shown in FIG. 1B, a collar insulating film 4 is formedin a well-known method, and thereafter, a capacitor insulating film 5 isdeposited on the entire surface. A silicon oxide film or silicon nitridefilm is used as the collar insulating film 4.

[0048] Next, as shown in FIG. 1C, a thin polycrystalline silicon film 6as a liner film is deposited on the entire surface such that the trench3 is not closed. The thickness of the thin polycrystalline silicon film6 is 20 nm, for example. A LPCVD method which is a kind of CVD methodwith excellent coating quality may be used as a film forming method.

[0049] The film forming condition for the thin polycrystalline siliconfilm 6 is as follows. That is, the temperature and pressure arerespectively set to 600° C. and 0.3 Torr, and silane (SiH₄) is used fora raw material gas.

[0050] Thereafter, as shown in FIG. 1C, a silicon germanium film 7containing phosphorus, arsenic or boron is deposited as a node electrodeon the thin polycrystalline silicon film 6, for example, by the LPCVDmethod such that the trench 3 is not closed. Although the silicongermanium film 7 is preferably amorphous, this film 7 may bepolycrystalline.

[0051] The relationship between the resistance rate (mΩ cm) of thesilicon germanium film 7 and the ratio of germanium differs depending onthe dopant to be applied. For example, as shown in FIG. 4, in case wherethe dopant is phosphorus, the resistance rate of Si_(1-X)Ge_(X) filmincreases rapidly if the composite ratio X exceeds 4.5. In contrast, ifthe dopant is boron, the resistance rate does not substantially changebut is maintained at a low value if the composite ratio X exceeds 1.5 orabove.

[0052] Therefore, if priority is given to reduction of the resistancerate, a silicon germanium film 7 applied with phosphorus and having agermanium composite ratio of 5.5 or less may be used. In order tofacilitate control of the application amount of a dopant, a silicongermanium film 7 applied with boron and having a germanium compositeratio of 1.5 or more may be used.

[0053] Next, as shown in FIG. 1D, the silicon germanium film 7 is flowedto fill the trench 3 by performing a heat treatment on the silicongermanium film 7. If an undoped silicon germanium film 7 is formed inthe step shown in FIG. 1C, a dopant is applied by ion implantation afterthe silicon germanium film 7 is flowed in the present step.

[0054] To flow the silicon germanium film 7 effectively, the temperatureand pressure in the heat treatment are set higher than those in the filmformation of the silicon germanium film 7. Specifically, the temperatureof the heat treatment is 750 to 950° C., and the temperature of the filmformation of the silicon germanium film 7 is 550 to 650° C.

[0055] If the film formation temperature and the heat treatmenttemperature are thus selected, an amorphous silicon germanium film 7 isformed in the step of FIG. 1C and is thereafter changed into, apolycrystalline silicon film 7 in the later step of FIG. 1D. Note thatthe silicon germanium film 7 can be flowed even if the temperature andthe pressure are set higher.

[0056] The amorphous silicon germanium film 7 excels the polycrystallinesilicon film 7 in the step coverage. Therefore, in case where the aspectratio of the trench 3 is as high as 10 or more like in the presentembodiment, the amorphous silicon germanium film 7 should preferably beformed first as described above.

[0057] To flow the silicon germanium film 7 effectively, the heattreatment may be carried out under the condition of the temperature andpressure described above in a state that an oxide film does notsubstantially exist on the surface of the silicon germanium film 7. Todo so, it is important that influence from oxygen and water vapor isreduced sufficiently. Accordingly, a preferable atmosphere for the heattreatment should be an atmosphere such as a hydrogen atmosphere whicheffects deoxidation or an inactive gas atmosphere of Ar or the like inwhich the partial pressures of oxygen and water vapor are sufficientlylow during the heat treatment. It is also preferable that the partialpressure of water vapor (P_(H2O)) is lower than 10⁻¹⁰ (=P_(H2O)/P_(H2))with respect to the partial pressure of hydrogen (P_(H2)). The smallerthe value of P_(H2O) is, i.e., the smaller the value of P_(H2O)/P_(H2)is, the more difficult the oxidation is.

[0058]FIG. 5 shows the dependency of the melting point of Si_(X)Ge_(1-X)(0≦x≦1) on the silicon ratio (H. Stroeand W. Klemm, Z. anorg. Chem.,vol. 24, 1939, p 305-323).

[0059] As shown in the figure, silicon (x=1) has a melting point of1412° C., and germanium (x=0) has a melting point of 940° C. Silicongermanium (x≠0, 1) has melting points of silicon and germanium.Therefore, the silicon germanium film can be flowed by a heat treatmentat a lower temperature than the silicon film.

[0060] Accordingly, in the heat treatment step shown in FIG. 1D, thethin polycrystalline silicon film membrane 6 acts as an appropriateliner film with respect to the silicon germanium film 7 and only thesilicon germanium film 7 can be flowed. As a result, the inside of thetrench 3 can be buried easily without causing a void, a crack (seam), orclearances 88 ₁ to 88 ₄ as shown in FIGS. 6A to 6C.

[0061] Thus, according to the present embodiment, the inside of a deeptrench 3 can be buried easily without causing a void, a crack (seam), orclearances, by coating the inner wall of the deep trench 3 with a thinpolycrystalline silicon film 6, depositing thereafter a silicongermanium film 7, and performing a heat treatment on the silicongermanium film 7. As a result, increase of the node resistance anddecrease of stored charges can be suppressed.

[0062] The present invention is not limited to the above embodiment. Forexample, the present invention is effective for a capacitor which doesnot have a collar insulating film.

[0063] Although only the case of a trench capacitor has been explainedin the above embodiment, the present invention is particularly effectivefor a trench capacitor having a deep trench with a high aspect ratio,e.g., a trench capacitor forming part of a memory cell in a DRAM.

[0064] Although the above embodiment uses a silicon germanium film,similar effects can also be obtained by using a germanium film. It isonly necessary that a conductive film having a lower melting point thana thin polycrystalline silicon film be used as a node electrode.

[0065] Although the above embodiment explains a case of applying thepresent invention to a trench of a capacitor, the present invention isalso applicable to a trench for STI (Shallow Trench Isolation).

[0066] In the above embodiment, deposition of a silicon germanium filmand a heat treatment thereof are carried out only once. The inside ofthe trench may be buried by repeating the deposition and heat treatment.

[0067] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A semiconductor device comprising: a siliconsubstrate having a trench formed in a surface of the silicon substrate;a polycrystalline silicon film coating an inner surface of the trenchsuch that the trench is not closed; and a conductive film made ofmaterial having a lower melting point than silicon and formed on thepolycrystalline silicon film so as to fill the trench.
 2. Asemiconductor device according to claim 1, wherein the conductive filmis a node electrode of a trench capacitor.
 3. A semiconductor deviceaccording to claim 1, wherein the material is either silicon germaniumor germanium.
 4. A semiconductor device according to claim 1, whereinthe conductive film is a Si_(1-X)Ge_(X) film (X>1.5) containing boron.5. A semiconductor device according to claim 1, wherein the conductivefilm is a Si_(1-x)Ge_(X) film (X<0.45) containing phosphorus.
 6. Asemiconductor device according to claim 1, wherein an aspect ratio ofthe trench is 10 or more.
 7. A method of manufacturing a semiconductordevice, comprising steps of: forming a trench on a surface of a siliconsubstrate; coating an inner surface of the trench with a polycrystallinesilicon film as a liner film such that the trench is not closed; forminga conductive film made of material having a lower melting point thansilicon, on the polycrystalline silicon film, such that the trench isnot closed; and flowing the conductive film so as to fill the trench, byperforming a heat treatment on the conductive film.
 8. A methodaccording to claim 7, wherein the conductive film is a node electrode ofa trench capacitor.
 9. A method according to claim 7, wherein thematerial is either a silicon germanium or germanium.
 10. A methodaccording to claim 7, wherein the conductive film is a Si_(1-X)Ge_(X)film (X>1.5) containing boron.
 11. A method for according to claim 7,wherein the conductive film is a Si_(1-X)Ge_(X) film (X<0.45) containingphosphorus.
 12. A method according to claim 7, wherein an aspect ratioof the trench is 10 or more.
 13. A method according to claim 7, whereinthe conductive film is formed in an amorphous state on thepolycrystalline silicon film and is thereafter changed intopolycrystalline after the heat treatment.
 14. A method according toclaim 7, wherein the heat treatment is at a temperature higher than atemperature at which the conductive film is formed.
 15. A methodaccording to claim 7, wherein a pressure of the heat treatment is sethigher than a pressure at which the conductive film is formed, andpartial pressures of oxygen and water vapor in an atmosphere duringformation of the conductive film and a heat treatment thereof are setlow than a main gas in the atmosphere, thereby to attain a state inwhich an oxide film does not substantially exist on a surface of theconductive film.
 16. A method according to claim 7, wherein anatmosphere during the heat treatment is either a hydrogen atmosphere oran inactive gas atmosphere.